Then you can't do it. Lion requires 2 GB of RAM.
What processor do you have in your iMac?
If you have an Intel Core Duo and not an Intel Core 2 Duo that will prevent you from going to Lion even if you upgrade the RAM.
There is no combo update that will get you there if your Mac can run Lion nor can you skip Snow Leopard.
You will need to buy Snow Leopard and upgrade to 10.6.8 in order to get the correct App Store for Lion.
Then you will need to purchase, download and install Lion from the App Store.